Functions: provide dummy multi function

Sometimes it is convenient to be able to return a reference to some
dummy function.

source/blender/functions/FN_multi_function.hh

@@ -88,9 +88,9 @@ class MultiFunction {
   }
 
  protected:
-  MFSignatureBuilder get_builder(StringRef function_name)
+  MFSignatureBuilder get_builder(std::string function_name)
   {
-    m_signature.function_name = function_name;
+    m_signature.function_name = std::move(function_name);
     return MFSignatureBuilder(m_signature);
   }
 };
@@ -100,6 +100,8 @@ inline MFParamsBuilder::MFParamsBuilder(const class MultiFunction &fn, uint min_
 {
 }
 
+extern const MultiFunction &dummy_multi_function;
+
 }  // namespace fn
 }  // namespace blender